Electron tube



April 26 1949- J. H. o. HARRIES 2,468,440

ELECTRON TUBE Filed Dec. 26, 1945 2 Sheets-Sheet l //7 Veni. on

April 26, 1949. J. H. o. HARRn-:s

ELECTRON TUBE Filed Deo. 26, 1945 2 Sheets-Sheet 2 F/Gj 655W di.

AKM/m65) hunted Apr. 2s, 1949A ELECTBON TUBE John Henry Owen Battles,London, England Application December 26, 1945, Serial No. 637,194 InGreat Britain January 4, 1945 (Cl. o-27.5)

3 Claims.

1 The present invention relates to electronic discharge tubes, and, inparticular, to discharge tubes suitable for operation at very highfrequencies.

It is known that in electronic tubes having a positive grid orequivalent electrode between a cathode and an anode to which an outputcircuit is connected, secondary electrons are emitted from the anode andthat, if during operation the anode potential becomes less than that ofthe said grid, these secondary electrons tend to travel from the anodeto the grid or equivalent electrode. This results in a reduction of thenet electron current flowing to the anode, and reduces the efllciency oftransfer of energy to the output anode circuit. q Furthermore, otherundesirable effects are also produced by this backward passage ofsecondary electrons.

One well known remedy is to interpose between the accelerating electrodeand the anode a low potential grid which then prevents the backwardpassage of secondary electrons. Such a grid is generally referred to asa suppressor" grid, and was described in Patent Specifications Nos.1,945,040 and 1,945,041, dated January 30,

Another method of preventing the backward passage of secondary electronsto the grid is also well known, and was first described in my PatentSpecifications Nos. 2,045,525, 2,045,526, and 2,045,- 527, dated June23, 1936. This will be referred to as the critical distance method.

Combinations of these two principal methods have also been suggested andused. Other methods exist such as the use of specially shaped anodes andanodes having a coating of graphite; but, though on occasions thesemethods may be combined advantageously with the two principal methodsmentioned above, they alone will not, as far as is known, remedy the badeffects of secondary electrons successfully.

The prevention of the undesirable effects of the passage of secondaryelectrons as distinct from the reduction of the number emitted will bereferred to as the process of suppressing secondary electrons.

In certain cases, the above remedies involve difficulties which preventtheir being employed. For example, in electron tubes operating at veryhigh frequencies, for example, frequencies higher than 3X 108 cycles persecond, it may be very difiicult to maintain a suppressor grid at thesteady potential which is necessary for it to operate correctly.Moreover, the size and configuration of the tube may be such that it isdillicult to introduce a suppressor grid at all. Again, in connectionwith the critical distance method, the distances and spacingsnecessarily used in tubes intended to operate at extremely highfrequencies are sometimes such that it is not possible to place theanode at or at about the critical distance.

It will be appreciated, therefore, that up to the present, there hasbeen no way of successfully preventing the deleterious effects ofsecondary emission, except those previously mentioned, and they are notsuitable for use at very high frequencies. In fact, hitherto, it hasbeen considered that, at very high frequencies, secondary electronscannot well be suppressed at all, and therefore it is common practice toavoid their production by arranging for the beam of electrons not tostrike either the anode, or the positive accelerating grid, though itmust of necessity pass through the eld between them. It is not easy toarrange this, and to do so increases the difficulties of manufacture toa serious extent.

It has now been discovered, during experiments with electron beam tubes,that in certain novel circumstances, the output power need not beadversely affected by the passage of secondary electrons, although nosuppressor grid is employed, and the anode is not at the erticaldistance" above-mentioned.

One object of the present invention is, therefore, to overcome thedeleterious effects of the backward passage of secondary electrons,especially in electron tubes operating at extremely high frequencies, bymeans not involving the disadvantages of the known methods ofsuppressing them.

Thus, according to the present invention, an electron valve or dischargetube is arranged to produce one or more beams of electrons, and is gfurnished with a pair of output electrodes lying in succession in thepath of the beam or beams, and with electrode means which can modulatethe beam or beams so as to energise an'output circuit of which theoutput electrodes form a part, while the pair of output electrodes arespaced apart by such a distance that, within some part of the ranges ofworking voltages and frequencies, the transit angle between the pair ofoutput electrodes lies within the optimum range as defined belowpeculiar to the surfaces and f configurations of the pair of outputelectrodes so that secondary electron emission from the second outputelectrode is substantially suppressed, and the power output efficiencyof the tube is substantially higher than it would be if the transitangle tended to zero. Such a tube may have the stream of electronsformed into a single beam, preferably having a. uniform current densityover its cross-section. The beam or beams formed should naturally besuch that all its constituent electrons have practically the sametransit angle between the two output electrodes. In addition, thesuppressor action may be substantially assisted by one of the knownmethods mentioned above; for example, a carbonised anode surface 3 or aslotted or recessed anode surface or both may be used.

It has now been found in practice that in the case of plane and polishedoutput electrode surfaces, secondary radiation may, particularly if thesurfaces are contaminated, increase so much that the maximum efficiency,even at the optimum transit angle, may not be enough for many purposes.It is therefore preferred to reduce the emission of secondary electronsfrom the electrodes as much as possible, by, for example, coating thesurfaces with colloidal graphite or corrugating or slotting them, orusing grids or electrodes with grid-like configurations or by utilisingmore than one of these expedients.

'The pair of output electrodes referred to consists of two electrodeslocated successively in the path of the electron stream, and adapted toform parts of an output circuit, so that during operation, anoscillatory electricl field exists between them when the electron beamis modulated. The configuration of each of this pair of outputelectrodes is such that the electric field between them is substantiallybounded and confined by them and does not appreciably extend throughthem. Of this pair of output electrodes, that through which themodulated electron current first passes is referred to as the firstoutput electrode, and that to which it then travels is referred to asthe "second output electrode or "anode and which has a configurationsuch that at least a substantial part of the electron stream strikes it.

Before explaining more fully the importance of transit angle between thepair of output electrodes in the present invention, it will beconvenient to refer to some particular examples of electron tubesaccording to the invention and methods according to which they may beused' and some such examples will now be described in more detail withreference to the accompanying drawings, in which:

Figure 1 is an elevation of an electronic tube, with the outputelectrode system arranged according to the present invention;

Figure 2 is a plan of the first output electrode seen from below inFigure 1;

Figure 3 is a set of curves showing graphically power output eiciencyplotted against transit angle;

Figures 4, 6 and 8 are sectional elevations showing certain alternativeconfigurations of anodes; while Figures 5, 7 and 9 are plan viewsof therespective forms; and

Figure 10 is a circuit diagram showing how the novel tube may beutilised.

In the example of an electron beam tube having an output system asillustrated in Figure 1, a beam of electrons travels along the path I,through the slot 2 in a first output electrode E,

to the anode, or second output electrode A. The

output electrodes E and A form part of an output circuit, or cavityresonator 3, which is of the concentric line type. The beam of electronsis produced and modulated by means situated within the envelope 4, andalthough this means may consist of any of many well known devices, aconvenient form illustrated in Figure 1 is that described in my patentapplication Serial No. 409,585, led September 4, 1941, now Patent No.2,396,949. As in that prior application, the cathode C, negative gridGI, positive grid G2, defiecting electrodes DI, D2 and electrodes L2 andS are provided and need: not be further de- 4 scribed here. However, thefirst output electrode E now takes the place of the sub-anode SA in thesaid prior application.

The anode A (Figure 1) which constitutes the second output electrode isprovided with a detachable face 5 which is secured to the back part 6 byscrews 1. The anode A as a whole is cooled by water which is arranged tocirculate in the inner conductor 8 of the cavity resonator 3 through theinlet and outlet pipes 9, Ill. The cavity resonator 3 is constructed intwo parts, namely the base II and the top I2. The flange I3 on the topI2 is removably bolted to the base II by bolts (not shown). The base IIis watercooled by water arranged to circulate in the circumferentialslot I4 in the base II. The joints between the flange I3 and the base IIarek made vacuum-tight by causing vacuumvwax I5 to flow and which mayconveniently be that known by the trade name Apiezon WAO. Similarly, theglass envelope 4 is made vacuum-tight and attached to the base I I bymeans of vacuum wax I6, which may conveniently be that known by thetrade name Apiezon W.100. The assembly will be recognized as one whichmay be demounted for the purpose of altering the internal arrangementsof the apparatus. On'reassembling, it may be sealed up and madevacuumtight by means of the Wax I5, I6.

The beam of electrons travelling along the path I will travel betweenthe pair of outputv electrodes through the slot 2 in the first outputelectrode E, and will strike the anode A.

It will be recognized that if the cavity resonator 3 as a whole ismaintained at a steady potential Vb, and if the beam of electrons ismodulated at a frequency which is equal to the resonant frequency of theresonator 3, then energy will be delivered from the beam of electrons tothe resonator 3, power will appear inside the resonator 3 and may be ledout of it into a load by means, for instance, of a loop 22 or other ofmany well known devices. The apparatus will be recognized as one capableof operating at a very high frequency.

When the pair of output electrodes E and A form part of an outputcircuit, if a modulated beam of electrons travels along the path I andpasses through the slot 2 in the first output electrode E to the secondoutput electride A, then, as already mentioned, power is delivered fromthe beam to the output circuit which is tuned to the modulationfrequency (f) of the beam. The pair of electrodes E and A are maintainedat a steady unidirectional potential Vp except for those variations inpotential due to the voltage across the output circuit which occurs whenpower is fed to that circuit.

It is usual to assess the performance of such a system by measuring itspower output efficiency (e) :P0/Pb Equation 1 where Pn=the power outputfed to the output circuit from the beam, and Pb=the direct current powerflowing from the source of voltage Vb.

T=1/f and is the periodic time ofthe modulation frequency of the beam.

If the distance between the pair of output electrodes E, A is d, then It=d/v Equation 2 =t/T.21r=21rf.t Equation 3 'Then substituting t=d/v andthe value of v:

` It is thus seen that the same power output efliciency (e) can beobtained for any given distance d provided that f/\/Vb is maintainedconstant although the actual values of f and Vb may vary widely. Infact, in theory, there are an innite number of combinations of differentvalues of d, f and Vb which will yield the same power output efficiency.Any electron tube to provide a given value (e) has therefore to bedesigned so that these three parameters satisfy Equation 4 for a valueof which corresponds to the particular efficiency (e) desired. Therelationship between and e is complex and, as far as is known, has notyet been clearly stated in the literature and it is not necessary toconsider it in further detail here.

Hitherto, however, it has been thought that if 4 is increased, e will bereduced and will, in fact, fall to a very low value when is greater thanof the order of 21r.

lIt has also been held hitherto that if the voltages upon both of theelectrodes E and A are equal, the presence of secondary electronemission from the second output electrode A when it is struck by primaryelectrons will reduce the power output efficiency (e) to a very lowvalue at all transit angles (.p)

It has now been discovered, however, that if the transit angle lieswithin a certain optimum range, secondary electrons are suppressed and agood power output efficiency is obtained. The position and extent ofthis optimum range `of transit angles depends upon the nature of thesurfaces and configurations of the particular pair of output electrodesE and A which are chosen.

Itwill be appreciated that the technique of electron tube design, andparticularly of those intended vto operate at very high frequencies, is:such .that only certain values of Vb, f and d in Equation 4 can, infact, be used because considerations of practical design rule out somevalues. Only certain values of d are, in fact, practical for mechanicaland other reasons. For a given desired power output, Vb cannot haveother cerltain values. Other known factors limit the possible values ofthe operating frequency f. Indeed, in certain electron tubes such asthose op erating by velocity modulation, the useful values of Vb and fmay be closely interlinked so that 'practically operative values coveronly very narrow ranges and the input electrode system is adapted tomodulate the electron stream over only a narrow band or bands offrequencies for any given set of operating voltages. Therefore,

Equation 4 Athe present invention can only be applied to tubesin-respectowhich the operating ranges of volt- *"7 5 age and frequencyare such that a value of d can be determined from Equation 4 which iswithin practicable limits, when the transit angle p has a value lyingwithin the said optimum range appropriate to the particular pair ofoutput elec.- trodes used, within which range, secondary electronemission is suppressed.

- Since the base Il and the rest of the cavity 3 are in conductivecontact and are therefore necessarily at the same potential, it wouldhave been thought hitherto as mentioned above that, secondary electronsproduced from the surface of the electrode A where it is struck by theprimary electrons, and from the edges of the slot 2 (if any of theelectrons strike them) would cause the power output efficiency of thedevice to be so low as to be almost useless, probably only one or twoper cent, or even less.

Nevertheless, forthe reasons explained above, in accordance with thepresent invention, the power output efficiency in fact is arranged tohave a useful value by constructing the valve so that the distance d inFigure 1 is such that the transit angle is in the neighbourhood of theoptimum value (m) at which secondary electrons are suppressed.

It has now been found that the optimum transit angle (en) in fact,varies according to the mate',-- rial and configuration of the outputelectrodes (anode A and the first output electrode E), and

therefore that (ou) can best be found for any one of the many possiblematerials and configurations by one of the following methods.

In following the rst method, a special test tube is made which has a.mechanically movable anode A. For example, the tube illustrated inFigure 1 may be looked upon as such a test tube. If the assembly isdemounted, the anode A may be moved to different positions so as to givedifferent values of d. Apart from any such provisions made to move theanode A, the test tube should preferably be identical with the nal tubein which it is desired to utilise the present invention. Test voltagesare applied to the anode A and to the output electrode E of the testtube which voltages are preferably equal to or in any case, not widelydifferent from those at which the final tube is to operate. 'I'he beamof elec.- trons is produced and modulated at various frequencies over arange which includes the fre-` quency fo at which the final tube is toWork. Thev output power efficiency of the test tube is then measured ata number of different distances between the output electrodes and atthese frequencies.

A curve is plotted showing the relation between the transit angle o andthe power output efficiency (e) as illustrated in Figure 3. Asv wasfirst shown Joy the research upon which the present invention is based,this curve will be found to exhibit an optimum range of values of theelciency (en) where it is high compared with its value when the transitangle tends to zero.

In accordance with the present invention, a final tube is thenconstructed so that, at an operating voltage Vb of the first outputelectrode E, and at an operating frequency fo, the transit angle iswithin the optimum'range, and therefore sec'- ondary electrons from thesecond output electrode A are suppressed and the power output efficiencyis high. Y

In the second method, instead of using a test tube having movableanodes, a family of test ftubes may be used. All of them are to beidentical except that -the distance between the pair` of outputelectrodes E and A varies from one tube to another.

The third method is used for the test if, for instance, the means usedfor modulating the beam in the final tube is such that it will notoperate over a range of frequencies. The conflgurations and materials ofthe output electrodes E and A must be, of course, identical with thoseused in the nal tube.

In short, the range of optimum transit angles for the nature of surfacesand configuration of a given pair of output electrodes E and A arrivedat by means, for example, of the tube illustrated in Figure 1, may beapplied to other tubes by the use of Equation 4 in which the materialsand con i'lgurations of the output electrodes are the same. Utilisingthis tube as shown in 'Figure 1, the measurements shown graphically inFigure 3 were made. The parts of the target A were interchanged betweenthose illustrated in Figures 4 to 9.

Curve R was obtained when the surface of the part 5 of the anode A wasof polished copper as shown in Figures 4 and 5. It will be observed thata maximum elciency was obtained of ap.

proximately 28% at a transit angle o=0.51r and is much greater than thevalue when tends to 0. Final tubes can be made with their respectivedistance d computed so as to give transit angles with their respectiveoperating values of f and Vb. As previously pointed out such a planepolished copper surface as that of part 5 (Figures 4 and 5) is found tobe particularly prone'to reduction of the maximum eiciency obtained,even at the optimum transit angle m=0.5, should it. for eX- ample,become contaminated by barium from a cathode coating or from a getter.An improve'- ment may be obtained by roughening the surface and/orcoating it with colloidal graphite. Care should, however, always betaken to avoid contamination of the surface by any substance of the manyknown to increase the number of secondary electrons emitted.

Curve S is that for the anode face with its copper surface of part 5roughened by sandblasting and coated with colloidal graphite as inFigures 6 and 7. It is well known that such treatment will reduce theemission of secondary electrons. The maximum eiciency (en) is now about31%, and again occurs at a transit angle (ou) =0.51r and holds over aconsiderable optimum range.

As previously mentioned, a better power output efficiency, which tendsnot to be particularly affected by contamination and like defects isproduced if the surface of the anode A is recessed or slotted so thatmost of the electrons in the stream strike that surface at the bottom ofthe recesses or slots. In exempliication of this, a copper anode (thepart 5 in Figures 8 and 9) is used, which is slotted, sandblasted, andcoated with colloidal graphite. The high frequency field in the cavity 3is believed not to extend appreciably into the slots, therefore thoseelectrons which pass into the slots I1 pass substantially out of thefield. Curve T inFigure 3 is then obtained. There is a substantial-advantage gained by slotting the anode surface inasmuch that a maximumefficiency of as much as 46% is then obtained at a transit angle of (dm)=0.31r and is again maintained siderable optimum range.

This improved result may be explained by the following hypothesis.Secondary electrons are suppressed in accordance with the presentinvover a con- 8 vention because at the instant when they are emittedthe electric field at the surface of the anode A (Fig. 5) is directedtowards the anode and normally to its plane. This is a result of thetime of an'lval of the electrons in the stream compared with thevariation of the electric field with time. Therefore normally emittedsecondary electrons will tend to be returned to the anode by thenormally directed electric field; that is, they will be suppressed.Those secondary electrons which are not emitted normally .to the anodeplane, that is, those which are emitted in directions other than that inwhich the electron stream travels will not be so readily returnedharmlessly to the anode by the electric field and will not therefore besuppressed so effectively.A If the surface of the anode is slotted(Figures 8 and 9) so thatmost of the electrons in the stream strike thesurface at the bottom.

of a slot, then many of these secondary electrons that are not emittedin the direction of the electron stream, but travel sideways whenemitted, are caught by the sides of the recess or slot before theytravel into the electric iield between the anode A and the'electrode Eand are therefore rendered harmless.

'I'he above results were obtained when the beam of electrons wasmodulated virtually completely with a. sine-shaped wave, and the figuresof efiiciency refer to that part of the beam of electrons which actuallytravels through the slot 2 into the interior of the resonant cavity 3.

The surface of the electrode E was sandblasted and coated with colloidalgraphite since such treatment was found to be advantageous.

In Figure 3 the parts of the curves marked with crosses are unstable,and of no interest for the present purposes. v

It will be appreciated that, in some forms of the invention, the twooutput electrodes E and A may not necessarily be maintained duringoperation at the same voltage Vb. For example, in Figure 1, a sheet ofmica may be inserted between the base H and the ange I3 of the cavityresonator 3 to separate them for direct current voltages, so that therst output electrode E and the anode A may be maintained at differingdirect current potentials.

In Figure 10 an example of a connection for a transverse control valveof the kind described in my Patent 2,396,949 is shown. 'I'he suppressorgrid S is directly connected to the cathode C by a vconductor I8. Theelectrode L2 and the cavity resonator 3, which includes the anode A andthe accelerating electrode E, are connected by conductors I9, 20 to aIbattery B1 of 2500 volts; the positive grid G2 is connected by aconductor 2| to a tapping at 1400 volts potential in the battery B1. Theshield grid G1 lis connected to a negative biassing battery B2. Thecathode C is heated by the battery B3. The deflection plates 13|, D2,are connected through a transmission line Tl, T2, to a source G ofhigh-frequency voltage. A beam of electrons I produced from the cathodeC is transversely deected dury ing operation by the deflection platesDI, D2, so

that it is moved in an oscillatory manner over the bottom surface of theelectrode E, so 'as to produce periodic pulses of electrons through theslot 2. These pulses travel through the slot 2 in the rst outputelectrode E to the second output electrode, or anode A, in the cavityresonator 3. The distance d between the output electrodes (anode A andrst output electrode E) is such that at some part of the working rangeof operative voltages and operative frequencies of the apparatus, thetransit angle between the output electrodes lies Within an optimum range,over which secondary radiation is substantially suppressed.

Energy is withdrawn from the cavity resonator 3 by means of the probe 22and fed by a transmission line 23 to an aerial 24.

More than one pair of output electrodes may be used, in accordance withthe invention, in a given electron valve.

I claim:

1. An electron discharge device comprising means including a cathode forproducing an electron stream, an input electrode system for modulatingsaid electron stream at high frequency, an output electrode located inthe path of said stream, a second output electrode located in the pathoi said stream and spaced from said rst output electrode on the oppositeside thereof from said cathode, a resonant output system connectedbetween said output electrodes, said second output electrodeconstituting the principal collector electrode for said electron streamand having a collector surface treated to avoid excessive secondaryelectron emission resulting from a contaminated metal electrode surface,and said second output electrode being spaced from said iirst outputelectrode a distance corresponding to a. transit angle between saidoutput electrodes, for a given operating voltage and frequency ofoperation, oi lese than about 0,81- radians and is at 10 or near themaximum of the curve relating output power efficiency of said outputresonant system to the transit angle between said output electrodes.

2. An electron discharge device according to claim 1 in which theelectron receiving area. of said second output electrode is formed withrelatively deep recesses therein for receiving said electron stream, andwherein the walls of said recesses trap secondary electrons emitted fromsaid second output electrode at angles other than that along which saidstream travels.

3. An electron discharge device according to claim 1 wherein the transitangle between said output electrodes lies between 0.251 and 0.71rradians.

JOHN HENRY OWEN HARRIES.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,138,162 Hansell Nov. 29, 19382,225,447 Haei et al. Dec. 17, 1940 2,257,795 Gray Oct. 7, 19412,369,749 Nagy et al. Feb. 20, 1945 2,392,379 Hansen Jan. 8, 19462,396,949 Harries Mar. 19, 1946

